1. Field of the Invention
This invention relates to a method and apparatus for an anodic encasement sleeve for preventing the corrosion of metallic materials particularly ferrous metals) of underground storage tanks, most particularly directed to situations in which the tank is buried in a corrosive soil. This invention accordingly has application to buried tanks and tank systems, and any other buried storage systems that contain components of iron, steel, or other corrosive metal, such as concrete reenforcing wires, cylinders, rods, and cables.
2. Description of Related Art
The prior art regarding anti-corrosion measures in corrosive environments can be most simply divided into cathodic protection and environmental barriers. Cathodic protection recognizes that much corrosion is due to electrolytic processes occurring in the soil; such protection seeks to override, reverse, or manipulate these electro-chemical forces in an effort to forestall the corrosive effects of such forces. Environmental barriers, more simply, employ electrolyte-impermeable materials to encase the metallic structure sought to be protected. Rather than manipulating the electrical properties, the environmental barriers attempt to electrically isolate the tank, cutting the electrical circuit altogether.
Cathodic protection manipulates the electro-chemical circuit occurring in the electrolyte (e.g., the soil) generally by one of two methods: impressed current or sacrificial anode. Both the sacrificial anode method and the impressed current method rely on altering the electrical exchange that is occurring in the electrolyte. When impressed current is employed, the tank is essentially connected to an external power supply. Because this external power supply expresses greater electrical potential than that possessed by circuit between the electrolyte and the tank, the electrochemical decay of the tank is controlled. A primary disadvantage of the impressed current method is its cost. On the front end, impressed current systems normally cost multiple times the expense of a barrier technology to design and install. Additionally, throughout the life of the tank, the impressed current must be continuously expended; in other words, the electrical bill represents a continuing expense. Finally, impressed current systems can be overridden by nearby structures shorting the circuit or interfering with the operation of the current in other ways. To account for such possibilities, impressed current systems require continuing monitoring and maintenance by trained personnel for the life of the system. The impressed current method thus represents a technically workable solution that is prohibitively wasteful and expensive.
Sacrificial anodes, in contrast to the impressed current method, harness the differences in electromotive properties among metals. By conductively attaching to the tank an anode constructed of a metal with a higher electromotive potential than the tank metal being protected, the anode will decay in preference to the tank. Rather than introducing an external electrical field, as in impressed current systems, the sacrificial anode becomes an internal component of the soil/tank electrical system. Disadvantages of the sacrificial anode approach include the ultimate exhaustion of the anode; once the anode has been consumed by corrosion, it leaves the tank unprotected. Furthermore, sacrificial anodes are notoriously inefficient. Estimates indicate that between approximately fifty per cent (for magnesium) and ten per cent (for zinc) of the sacrificial anode is lost due to "self corrosion" (e.g., the anode would corrode even in absence of its connection to the tank/soil circuit). Like impressed current, sacrificial anodes can be overridden by the effect of other structures or electrochemical systems in the soil. Accordingly, cathodic protection in its current forms represents an imperfect and wasteful solution to the problems of dependable corrosion protection over the life of a tank.
Given the waste inherent in sacrificial anode systems, not only do they suffer from inefficiencies, their useful life is limited by such wasting processes. Useful life is a critical factor in the selection of a tank material. The typical life of a cathodic protection system, however, is only approximately 20 years, requiring costly maintenance, replacement, and monitoring. Such activities further contribute to the waste inherent in the prior art cathodic systems.
The environmental barrier method offers a simple alternative to cathodic protection. Exemplary patents demonstrating use of environmental barriers include Wong et al., High Performance Composite Coating, U.S. Pat. No. 5,300,336; and Samour, Method of Coating Pipe, U.S. Pat. No. 4,211,595. Such barrier technologies maybe as simple as trench backfill procedures in which inert sand or other material beds a pipe, or they may employ organic or inorganic bonded protective coatings adhered directly to the substrate pipe. Numerous problems with these bonded barrier technologies, however, continue to plague the ferrous pipe industry. Particularly, it is commonly recognized that organic protective coatings will deteriorate with time, depending on the coating utilized, surface preparation employed, application techniques, temperature experienced, and environmental conditions tolerated. Furthermore, in real-world conditions, a defect-free bonded coating is impossible to economically obtain; where voids ("holidays") caused by application errors or by installation damage exist, corrosion will take place. If bimetallic or stray current corrosion conditions exist, localized corrosion will be accelerated at these holidays, causing corrosion failure faster at the discrete point of holiday than if no coating had been applied to the tank. This is because the corrosion forces are cumulated at the discrete area of holiday, rather than being distributed along a more substantial surface area of the tank.
In addition to these performance-based shortcomings, such bonded coatings are (like cathodic protection systems) economically wasteful. Preparation may be exacting and expensive; frequently tanks must be blast cleaned to a near-white state to ensure sufficient adhesion of a coating. Time spent preparing the coatings at a manufacturing facility is sub-optimally spent, because coatings will inevitably be damaged in the harsh construction transportation and installation environment. An improved protection system must therefore be forgiving of rough handling and easy to repair in the field. On the other hand, preparation of a coating on-site demands substantial delays caused by not only the coating process itself, but also by weather conditions and by any cooling or curing periods required.
Recognition of these performance and economic disadvantages in respect of ductile iron pipe has lead to the development of a polyethylene encasement solution as applied to pipe, which is the subject of international standard ISO 8180 and U.S. standard ANSI/AWWA C105/A21.5. Such polyethylene encasement does not require bonding of the barrier to the pipe; rather, the encasement is installed as a "sleeve" that draws over the sections of pipe like a tubular jacket (known in the art as "encasement" or "encasing" the tank). Installation at the site is quick and efficient, requiring no special training or curing times. Of all of the above listed methods for corrosion control and protection of pipe, polyethylene encasement has been the most successful, most economical, and most widely used method of corrosion protection for iron pipes. Since its invention in 1951, polyethylene encasement has been specified by more than 1,200 engineers to successfully protect more than 10 million feet of gray and ductile iron pipe and fittings. However, even with this method's tremendous success, failures continue to occur under real-world conditions due to the realities that damage and improper installation are inevitable.
In an attempt to harness the advantages of both cathodic protection and barrier technologies, numerous attempts have been made to combine the two. This is typically done by laminating the tank with sacrificial coating, followed by a bonded outer layer of polyethylene or some other bonded barrier coating. Exemplary use of such solutions is treated in Kemp, et al., Sacrificial Metal Pipe Coatings, U.S. Pat. No. 3,260,661. Kemp teaches that bonding to a pipe a laminated coating comprising a dielectric environmental protective layer and sacrificial anode foil may obtain the advantages of both cathodic protection and barrier technology. Notably, Kemp exemplifies the understanding in the art by teaching that the laminates must be firmly adhered to the pipe to avoid seepage or capillary action that may draw electrolytic solutions between the pipe and the closest layer of the laminate in the event of a holiday. The rationale commonly taught in the art is that if the laminate is not securely bonded to the tank, a puncture to the environmental barrier may allow corrosion to occur beneath the environmental barrier at the intersection of the sacrificial anode, the electrolyte, and the tank, where corrosion by-products interfere with the cathodic protection and accelerate corrosion. Bagnulo, Method of Corrosion Protection, U.S. Pat. No. 4,496,444, similarly teaches the need to have a tight bond between the entire sacrificial anode layer and a pipe. Bagnulo employs a sacrificial anode layer continuously bonded to the pipe by an electrically conductive adhesive.
The problem with such adhesive-bonded systems is that the electrolytic process that occurs at a holiday may produce hydroxyl and hydrogen gas ions, which deteriorate the adhesive bond, allowing seepage and capillary action to draw electrolyte beneath the coating. Samour, U.S. Pat. No. 4,211,595, discusses this propensity in regard to the prior art of pipe protection. When the electrolyte is drawn beneath the closely bonded coating, the adhesive itself may interfere with the electrical circuit, preventing the proper cathodic reaction. Additionally, at relatively large areas of damage to the sacrificial anodic layer, the relatively small exposed surface of the anode (generally only the width of the bonded anodic layer, multiplied by the circumference of the tear) will relatively quickly passivate, reducing or eliminating its effectiveness for preventing corrosion. Beyond the activity of the relatively small exposed surface of the anode, the pipe is no longer protected, and is subject to accelerated localized corrosion, as occurs at holidays in the total absence of cathodic protection.
Similarly, Shutt, Method and System for Protecting Corrosible Metallic Structures, U.S. Pat. No. 3,354,063, discloses an electrically conductive bonded sacrificial coating that is constructed by including anodic material into a binder. The binder/anode eventually passivates on its exposed surfaces, to form an environmental/electrical barrier in situ. Shutt contemplates the use of special backfill chemicals to effect the passivation of the bonded coating and separate chemicals to influence the non-passivation of external sacrificial anodes. This disclosure falls prey to the above mentioned problems of small anodic surface area activity, especially where passivation chemicals may seep into the holidays along with the electrolyte. Furthermore, the special backfill procedures add to the complexity and expense of installation and the propensity for errors in handling and installation.
Other anode bonded coating systems employ arc-applied zinc to apply the sacrificial anode layer directly to a pipe, avoiding the above-mentioned potential for undercutting the adhesive. Such arc applications are initially expensive in terms of specialized application equipment, surface preparation, expertise required, and time delay. The expense of such applications is further exacerbated by the inefficiency of the arcing process, whereby up to fifty per cent of the sacrificial anode material may be lost. These arc applications, therefore, do not solve the problem of wasted material and inefficiency. Furthermore, the zinc coating is normally topcoated with a bonded impermeable polymeric layer which restricts the surface area of anode that is available to protect a holiday and is similarly limited as in the adhesively applied anode layers. This is because if mechanical damage results in a holiday on the anode, the area of damage to the anode may be relatively large (e.g., a matter of centimeters or inches). Because the anode is tightly bonded to the tank on the one side (e.g., to avoid seepage), and bonded to the environmental barrier on the other side (e.g., to prevent the initial entry of electrolyte), the only surface area available for anode activity is at the edges of the holiday, which may present a surface depth of no more than 0.001 inch in thickness. This surface area maybe quickly passivated, again leaving no active anode, even though the tank may be copiously coated along its entire length with perfectly good anode material. This expensive anode material remains worthlessly idle because of the intentionally tight bonding to the tank and the other bonded layers.
A critical unifying feature of the prior art teaching and embodiments is that the sacrificial anodic layer and environmental barrier must both be bonded in succession to the pipe, or, in the current case, the tank. The art teaches that space between the tank and the first coating layer (whether the sacrificial anode or a barrier layer) is to be avoided because seepage and capillary action may draw electrolytes into unprotected contact with the tank, leading to accelerated corrosion. This teaching has lead to the conclusion that use of unbonded sacrificial anode layers in conjunction with polyethylene "sleeve" encasement is not only unworkable, but is counterproductive. Furthermore, the use of external sacrificial anodes located outside (i.e. the soil side) of polyethylene "sleeve" encasement has been hindered (a) by the "self corrosion" of the anode material itself, (b) by difficulties in obtaining uniform protective current distribution along the length of the tank (which itself exhibits resistance), (c) by the added expense of welding external sacrificial anodes to the tank, and (d) by allowing for passages of the connectors through the sleeve without loss of environmental protection, as well as numerous other difficulties that shall be apparent to those skilled in the art.